Conventionally, in order to enhance the performance of automobile tires, belt conveyors and the like, for example, a steel cord or the like that is plated with brass has been used as a reinforcing material. In order to improve the adhesive force between the reinforcing material and natural rubber or synthetic rubber, the rubber contains an adhesion promoter. As the adhesion promoter, an organic acid cobalt metal soap (for example, cobalt stearate, cobalt naphthenate, cobalt tallate, cobalt boron metal soap, or the like) has been frequently used because of favorable adhesive properties with the steel cord and the rubber.
However, cobalt compounds such as the aforementioned organic acid cobalt metal soaps are listed in Group 2B which is said to be “possibly carcinogenic to humans” in the list of carcinogenic risks classified by the International Agency for Research on Cancer. In addition, since metallic cobalt which is a raw material of various cobalt compounds is a rare metal, its supply is unstable. As described above, cobalt compounds (organic acid cobalt metal soaps) using a raw material which is suspected to be carcinogenic and also unstable in supply tend to be avoided although the adhesive properties with rubber and the metal (steel cord) are favorable, and there is a demand for alternative adhesion promoters (non-cobalt based adhesion promoters).
As a non-cobalt based adhesion promoter, for example, an adhesion promoter containing boron or phosphorus has been known. More specifically, for example, an adhesion promoter having a structure containing three atoms of nickel or bismuth bonded to boron or phosphorus via an oxygen atom and having both a residue of an aromatic carboxylic acid and a residue of an aliphatic carboxylic acid has been known (for example, see Patent Document 1). However, the adhesion promoter disclosed in Patent Document 1 has a problem in that the adhesive force when adhering the rubber and the metal is not sufficient.